Lithium ion secondary batteries are lighter in weight and have higher capacity than nickel-cadmium batteries, nickel metal hydride batteries, etc.; therefore, the lithium ion secondary batteries have widely been used as the power source for portable electronic appliances. Moreover, the lithium ion secondary batteries are the strong candidate for the on-vehicle power source used for hybrid vehicles or electric vehicles. In recent years, with the size reduction and the increase in functionality of the portable electronic appliances, the lithium ion secondary batteries used for the power source thereof has been anticipated to have higher capacity.
In view of this, an alloy-based negative electrode material including silicon or silicon oxide has attracted attention recently. Silicon is capable of electrochemical intercalation and deintercalation of lithium ions, and enables charging/discharging of much higher capacity than graphite. In particular, it has been known that silicon has a theoretical discharge capacity of 4210 mAh/g, which is approximately 11 times as high as that of graphite.
In general, the potential of the negative electrode active material is decreased during the charging and the side reaction of reducing and decomposing the electrolyte occurs. In this case, a film containing Li from the decomposition product of the electrolyte or the gas such as carbon dioxide or hydrogen is formed on a surface of the negative electrode active material. The generated gas increases the internal resistance of the lithium ion secondary battery and the lithium in the film no longer contributes to the charging/discharging; therefore, the discharge capacity decreases and the irreversible capacity is increased. By the formation of this film, the reaction between the electrolyte and the negative electrode active material during the charging is relieved and the lithium ion secondary battery is thus stabilized. The excessive growth of the film, however, is not preferable because the internal resistance is increased.
In the case of using silicon or silicon oxide for the negative electrode active material, silicon or silicon oxide is reduced in size in the charging/discharging cycle and the surface not provided with the film is formed in the charging/discharging cycle; in this case, it is difficult to stabilize the lithium ion secondary battery and the side reaction with the electrolyte easily continues in the charging/discharging cycle.
For suppressing the side reaction between the negative electrode active material and the electrolyte and improving the cycle characteristic, it has been suggested to coat the surface of the negative electrode active material of a lithium ion secondary battery containing a carbon material with a polymer film containing the alkali metal salt (for example, Patent Document 1) and to coat a silicon material with a polymer film or the like (for example, Patent Document 2).